I. Field of the Invention
The present invention generally relates to methods for foam cleaning combustion turbines by forcing a mist comprising a cleaning solution through the turbine. More specifically, the present invention is directed to methods for cleaning contaminants adhering to the internal surfaces of the compressor, combustion and turbine sections of a combustion turbine by forcing one or more cleaning solution mists therethrough. Also discussed is a manifold for partially blocking the air intake opening of the compressor section of a combustion turbine to facilitate such cleaning.
II. Description of the Background
Combustion turbines are used in a multitude of applications, including aviation, shipping, chemical processing and power generation. In combustion turbine power generation facilities, efficiency can be improved by supplementing the electrical power generated directly from the combustion turbine with recovery units designed to capture heat from the exhaust gas generated by the turbine. This heat can be used to produce steam to drive a steam turbine, operate steam driven equipment or provide heat to chemical processing facilities, thus improving the efficiency of the power generation facilities.
As used herein, the term combustion turbine refers to any turbine system having a compressor section, a combustion section and a turbine section. The compressor section is designed to compress the inlet air to a higher pressure. Atomized fuel is injected into the combustion section where it is combined with the compressed inlet air and oxidized. Finally, the energy from the hot gasses produced by oxidation of the fuel is converted to work in the turbine section. While fuels typically comprise natural and synthetic gases (mostly methane), other hydrocarbons, including liquified natural gases (LNG), butane, kerosene, diesel and fuel oils may be employed. The expanding combustion gases power the turbine by turning the rotating blades of the turbine sections as they escape the combustion section. The compressor section is mechanically powered by a rotor comprising a rotor shaft with attached turbine section rotating blades and attached compressor section rotating blades. In power generation facilities, the rotor drives an associated electrical generator. Alternatively, the rotor may be used to power chemical process equipment. While the exhaust gas may merely be discarded, preferably it is recovered as additional heat energy often being used to produce steam in power generation facilities.
The overall efficiency of a combustion turbine engine is heavily dependent on the efficiency of the compressor. The pressure ratio of the compressor, i.e., the ratio of air pressure at the compressor outlet to air pressure at the air inlet, is one of the significant parameters which determines the operating efficiency of the compressor. The higher the pressure ratio at a given rotational speed, the greater the efficiency. The higher the air pressure at the outlet of the compressor, the greater the energy available to drive the turbine downstream of the compressor and hence to generate power or produce thrust.
In axial flow compressors, pressurization of air is accomplished in a multiplicity of compressor stages or sections, each stage being comprised of a rotating multi-bladed rotor and a non-rotating multi-vaned stator. Within each stage, the airflow is accelerated by the rotor blades and decelerated by the stator vanes with a resulting rise in pressure. Each blade and vane has a precisely defined airflow surface configuration or shape whereby the air flowing over the blade or vane is accelerated or decelerated, respectively. The degree of air pressurization achieved across each compressor stage is directly and significantly related to the precise air foil surface shape. Unfortunately, the surfaces of the compressor blades and vanes become coated with contaminants of various types during use. Oil and dirt sucked in through the air intake become adhered to the blade and vane surfaces of the compressor.
Deposits build up on compressor blades during normal operation causing reduced airflow through the compressor section of combustion turbines. Such deposits are often the result of the ingestion of hydrocarbon oils and greases, smoke, dust, dirt and other particulate air pollutants through the air intake of the combustion turbine. Upon formation of a hydrocarbon film upon the internal surfaces, including both the rotating blades and stationary vanes, of the compressor, additional particulates pulled through the compressor become trapped. As the airflow through the compressor section diminishes, the compressor discharge pressure drops, resulting in a reduction in compressor efficiency and power output from the turbine. The resulting inefficiency causes an increase in fuel consumption and a loss in power generation output.
Aluminum and other metal substances erode from other parts, e.g., clearance seals of the engine, and are also deposited on the blades and vanes. Metals contained in the fuel, particularly heavy metals such as magnesium and vanadium, deposit on the combustion and turbine blades and vanes. All of these surface deposits alter the ideal air foil surface shape, disturbing the desired air flow over the blades and vanes. This results in a reduction in the pressure rise across each successive turbine stage and a drop in overall turbine efficiency.
Gas turbine compressors have been periodically cleaned to remove the build up of particulates on internal components. Some of this cleaning has been performed without full shutdown of the combustion turbine, while other cleaning methods have required not only full shutdown, but even disassembly of the turbine. Materials used in such cleaning operations have included water, ground pecan hulls, coke particles and chemical cleaning mixtures which have been sprayed, blown or otherwise injected into the inlet of the combustion turbine after it has been configured for such a cleaning operation.
Removal of contaminants from the blades and vanes of in service compressors is desirable to restore compressor and engine efficiency. Since it is both time consuming and expensive to disassemble the engine, methods capable of removing these contaminants without disassembly of the engine are desirable. Furthermore, any method utilized to remove the contaminants must not interfere with the structural or metallurgical integrity of the components of the engine. Acceptable methods must be capable of removing the contaminating materials without attacking engine components constructed of similar materials. Because many liquid solvents also attack the engine components, the injection of liquid solvents into the engine has often proven to be unacceptable.
Abrasive particles impinging upon the contaminated surfaces will also dislodge contaminants. However, abrasive materials have proven to be unsatisfactory. Such materials are often overly abrasive, not only dislodging contaminants but also destroying the surface smoothness of the blades and vanes. Furthermore, some of these abrasive materials generally remain within the engine. If non-combustible, these materials may clog cooling holes of the turbine components and restrict needed cooling airflow. If combustible, these materials may produce residues which clog the cooling holes.
A general discussion of compressor section cleaning may be found in Scheper, et al. xe2x80x9cMaintaining Gas Turbine Compressors for High Efficiency,xe2x80x9d Power Engineering, August 1978, pages 54-57 and Elser, xe2x80x9cExperience Gained in Cleaning the Compressors of Rolls-Royce Turbine Engines,xe2x80x9d Brennst-Warme-Kraft, September 5, 1973, pages 347-348. Several exemplary prior art cleaning methods are described in more detail below.
Many prior art methods merely sprayed water into the air intake of an operating combustion turbine. U.S. Pat. No. 4,196,020 to Hornak, et al. discloses a wash spray apparatus for use with a combustion turbine engine. The apparatus includes a manifold having a plurality of spray nozzles symmetrically disposed about the air intake of a combustion turbine engine. Water is sprayed under pressure from these nozzles into the inlet of the compressor during operation. The inlet air is used to carry the atomized water mist through the turbine. Some of the deposits, generally those at the front of the compressor, are contacted by the water and washed away, resulting in some improvement in efficiency. A similar system is disclosed by McDermott in U.S. Pat. No. 5,011,540. The McDermott patent discloses a manifold having a plurality of nozzles for mounting in front of the air intake of a combustion turbine. McDermott proposes that a cleaning solution be injected into the air intake as a cloud dispersed in the less turbulent air found at the periphery of the intake. McDermott asserts that dispersal in the less turbulent air improved cleaning. Similar water injection systems are available from turbine manufacturers for installation during construction of the turbine. Alternatively, these systems may be purchased as aftermarket items.
It has been observed, however, that water washes such as those described above only clean the first few rows of compressor blades and vanes. It is believed that this phenomenon is the result of both the high temperature and centrifugal forces generated in the operating compressor. These conditions cause the water to be thrown to the outside of the turbine and to be evaporated before effective cleaning throughout the length of the compressor section can be achieved. Further, water washes provide no benefit with respect to fouling occurring in the combustion and turbine sections of the turbine.
Attempts to improve cleaning efficiency resulted in the development of higher boiling cleaning solutions. For example, U.S. Pat. No. 4,808,235 to Woodson, et al. discloses cleaning fluids having relatively low freezing points, together with higher boiling points, to improve penetration and cleaning of the back rows of compressor blades. Woodson suggested that cleaning solutions comprising glycol ethers would provide improved cleaning throughout the length of the axial compressor. While addressing the evaporation problem, however, Woodson""s solution did not solve the problem resulting from centrifugal forces developed as the turbine spins during operation.
Other attempts to improve cleaning efficiency were directed to off-line methods. Systems similar to those just described were employed in conjunction with more rigorous off-line chemical cleaning procedures. During these operations, the unit is not fired. Atomized cleaning solutions, typically aqueous surfactant solutions, were drawn through the compressor by spinning the unit at a speed of about 1,000 RPM. While more effective than the previously described on-line cleaning procedures, the unit must be taken out of service, thus, increases costs through loss of output during the cleaning operation.
Some prior art systems employed abrasive particles in off-line cleaning. Unfortunately, non-combustible abrasive particles often clogged small cooling holes in the turbine blades, while combustible particles produced further residues on the blades. In an effort to overcome those deficiencies, U.S. Pat. No. 4,065,322 to Langford suggested that abrasive particles of coke having a carbon content of at least 70 percent-by-weight and a volatile matter content of less than 8 percent-by-weight be entrained in the inlet airstream and directed to impinge upon the contaminated surfaces. While these combustible coke abrasives avoided many of the problems found with prior art abrasive particles, they still did not provide a complete and full cleaning of the internal surfaces.
Accordingly, those skilled in the art have continued to seek improved methods for cleaning combustion turbines. Desirable methods should be capable of cleaning the blades and vanes throughout the length of an axial compressor and also of cleaning the blades and vanes in the combustion and turbine sections of the engine. Further, acceptable methods must not attack the engine components themselves. Thus, there has been a long felt but unfulfilled need for improved and more efficient methods for cleaning contaminants from combustion turbine engines. The present invention solves those needs.
The present invention is directed to methods for removing contaminants, including films, particulates, metals and other combustion products deposited in the compressor, combustion and turbine sections of a combustion turbine. In the methods of the present invention a mist comprising a cleaning solution is forced through the air intake of the compressor section and/or the fuel atomization nozzles of the combustion section of the turbine so that the misted cleaning solution contacts all contaminated internal surfaces of the turbine. The mist is forced through the turbine by a high pressure gas source. The present invention preferably employs a manifold for partially blocking the air intake of the compressor section of the turbine to facilitate the application of the driving force of the high pressure gas.
In the methods of the present invention, a conventional, liquid cleaning solution is prepared. Because the main contaminants in the compressor section are oils, greases and other hydrocarbons, along with entrapped dirt and dust, ingested with the air, an aqueous solution of a surfactant is preferred. The cleaning solution is introduced as a fine mist through the air intake into the compressor section of the combustion turbine by conventional mist nozzles. The cleaning solution may also be introduced to the combustion section through the fuel atomization nozzles.
The misted solution is forced through the turbine by applying a driving force supplied by a high pressure gas. While any non-reactive gas may be employed, compressed air provides the simplest and most economic driving force. A flow rate of about 1 to 2 linear feet per second through the turbine, resulting in a residence time of about 4 to 27 seconds in typical turbines, provides the best result. The solution penetrates all of the cavities of the compressor section of the turbine, thus bringing the solution into contact with contaminants covering all of the surface area of the blades and vanes disposed in the compressor section. Accordingly, a very thorough cleaning is obtained. After passing through the compressor section, the cleaning solution mist passes through the combustion and turbine sections where additional soluble contaminants are removed. Upon exiting the turbine section, the mist may be condensed to a liquid by conventional means. The liquid is then drained or removed from the turbine by any appropriate means.
In a preferred embodiment of the present method, a second mist comprising a cleaning solution suitable for removing contaminants produced during the combustion process and deposited on the internal surfaces on the combustion and turbine sections is prepared. These contaminants often include heavy metals and their oxides, together with shellacs, varnishes and other hard combustion residues. Accordingly, preferred cleaning solutions include aqueous solutions of an acid and a foaming agent. Suitable acids include both organic and inorganic acids. Particularly preferred are dilute solutions of the mineral acids. These solutions often include an appropriate corrosion inhibitor. Because the acidic cleaning solutions are typically not needed to remove contaminants from the compressor section and because these stronger solutions may attack and damage the vanes and blades therein, these stronger cleaning solutions should be prevented from entering the compressor section. In a preferred embodiment, this goal is accomplished by initially forcing a first, cleaning solution mist appropriate for cleaning the compressor section through the compressor section while forcing a second, stronger, cleaning solution mist through the combustion and turbine sections. Cleaning may continue until the prepared cleaning solutions are exhausted. However, in a preferred method, the recovered liquid is monitored for one or more contaminants, including metal content, to determine the progress of the cleaning. Cleaning is stopped upon reaching a predetermined value for the monitored contaminant.
The misted cleaning solutions described above provide acceptable cleaning results throughout a wide temperature range. While they typically function throughout the full temperature range where the underlying solution is liquid, i.e., from about 32xc2x0 F. to about 212xc2x0 F. for aqueous solutions, they are most often used at temperatures ranging from ambient, i.e., about 40-80xc2x0 F., to about 200xc2x0 F. Because the effectiveness of most cleaning solutions is improved at higher temperatures, most solutions are used at a temperature of about 150-180xc2x0 F. for maximum efficiency.
While the flow rate for these misted solutions can vary greatly, depending, on the initial pressure applied and the pressure drop through the turbine, the rate should be optimized to provide sufficient and complete contact while minimizing the overall cleaning time. The driving gas should be forced through the turbine at a flow rate of greater than about 0.2 linear feet per second, more preferably about 0.5 to 5.0 linear feet per second and most preferably about 1 to 2 linear feet per second. Those skilled in the art can readily calculate the linear flow rate based upon the internal volume of the turbine and the flow rate (cubic feet per minute) of the driving gas. These conditions should produce a residence time in the typical turbine (averaging 8-27 feet in length) of about no more than 40 to 135 seconds, preferably about 1.6 to 54 seconds and most preferably about 4 to 27 seconds.
While it is not necessary to turn the turbine during the cleaning operation, it is believed that contact with the misted cleaning solution is improved by slow cranking of the turbine. Accordingly, it is preferred that the turbine be turned at a speed not exceeding about 10 RPM, preferably about 5 RPM during the pumping operations.
At the conclusion of the cleaning operation any cleaning solution remaining within the turbine is easily removed by simply forcing a water mist through the turbine. Finally, the turbine may be dried before being returned to service by spinning at a speed and for a time sufficient to dry the turbine, typically about 500-1500 RPM and preferably about 1,000 RPM for about 10-30 minutes.
The manifold of the present invention provides a means for partially blocking the air intake opening of the compressor section of a combustion turbine. In an embodiment designed for use with an axial, bell-shaped air intake, the manifold comprises a bonnet for completely covering the air intake opening of the compressor section of the combustion turbine. The manifold includes means disposed about the periphery of the bonnet for producing a temporary seal with the air intake opening. Finally, the manifold includes at least one connection through the bonnet through which the misted cleaning solution and pressurized gas can be delivered through the air intake to the compressor section.
When installed, the manifold of the present invention provides a convenient means for confining the misted cleaning solution and forcing it through the compressor section of a combustion turbine to perform the cleaning methods of the present invention. By employing the cleaning methods of the present invention, contaminants in both the compressor section and in the combustion and turbine sections of a combustion turbine are conveniently and effectively removed in order to improve the compressor and turbine discharge pressures and the turbine power output, thus improving the efficiency and economy of the combustion turbine.
Thus, the long felt, but unfulfilled need for improved methods for cleaning combustion turbines has been met. These and other meritorious features and advantages of the present invention will be more fully appreciated from the following description and claims.